You might not know it, but a Halloween like the one coming up happens only once in a blue moon.
This October 31 offers a rare treat: not only a full moon on a night known for frights and fun, but the second full moon of the month, which is called a blue moon.
Think of how Halloween is depicted in films or art. The moon is almost always full, which in reality is rarer than it seems because a full moon on Halloween comes around only every 19 years. Add to the picture a blue moon, and this Halloween features a unique shine.
Let’s delve deeper into the moon and the interesting origins of the term blue moon.
First, the basics. The moon takes 29½ days to orbit Earth.
Although the moon seems bright, the light it produces is reflected primarily from the sun. Just like Earth, the moon has a day side and a night side. As the moon travels in its orbit, the dividing line between bright and dark— the terminator—is visible from different angles, making it appear different portions of the moon are lit up on different days. When the moon is completely lit, it appears full.
Since 1818, the Farmers’ Almanac has tracked moon cycles and dubbed the extra full moon in a year—with 13 full moons instead of 12—a blue moon. The title blue moon was given to the fourth full moon in a season when there are typically only three. According to that definition, blue moons could only happen in February, May, August and November. This means there never would have been a Halloween blue moon.
So, what happened? Amateur astronomer James Hugh Pruett, writing for a March 1946 edition of “Sky & Telescope Magazine,” misinterpreted the system of moon cycles in the Farmers’ Almanac and wrote that the second full moon in any month was a blue moon.
James’ definition stuck, especially after it was shared on the popular astronomy radio show “StarDate” in 1980 and adopted as an answer in an early version of the game Trivial Pursuit in 1986. Using the new definition, a blue moon comes around every two-and-a-half or three years. On rare years, when February gets no full moon at all, there can be two blue moons.
The origins of the well-known phrase, “Once in a blue moon,” predate the contemporary definition of the blue moon.
As early as 1528, people used the phrase much the way we say, “When pigs fly.” The complication with saying you will do something when the moon is blue—when you don’t plan to ever do it—is that sometimes the moon does actually appear blue. For example, during volcanic eruptions, larger particles of volcanic dust can diffract red light, which makes the moon appear blue. By the 1800s, the meaning of “Once in a blue moon” evolved from never to rarely.
Here are ways to make the most of this Halloween’s special treat.
Mary Kay Hemenway, now retired from the University of Texas at Austin astronomy department, says moon watching is one of the most accessible forms of astronomy.
While much stargazing requires dark skies, Mary Kay says that is not the case with the moon. A darker sky does, however, make the moon seem more dramatic.
Mary Kay says she likes to use binoculars to view the moon, but the full moon isn’t always the best time to view it. She suggests viewing partial phases to see the terminator, where the sunlit portion meets the shaded portion.
“Usually the surface looks rough, and you see the shadows of mountains and craters,” she says. “In fact, all those shadows look better at a crescent or quarter-phase because the light is coming from the side to make the shadows longer.”
Although you will have plenty of Facebook and Instagram friends trying, capturing memories of the moon on a cellphone camera is challenging.
“The moon is only a half-degree in diameter,” Mary Kay says.
“On my cellphone, it looks pretty tiny, so I haven’t tried taking pictures except when the moon accidently shows up in something else I was trying to get.”
To get good cellphone moon images, she suggests trial and error and a telescope.
“Holding the cellphone up to a telescope eyepiece gets you rather nice images once you figure out the proper distance to hold the phone to get it in focus,” she says.
If you have a more advanced camera, try these additional tips:
Pick a site with a clear view of the moon. Stake your spot out the night before to get an idea of where and when the moon will look best.
While you can shoot just the moon, consider adding foreground to give the moon perspective and add interest.
Bring a tripod to steady the camera.
Use a delayed shutter or shutter timer to reduce your camera’s wobble as you take the photo.
Full and Blue Moon Trivia
According to astronomer David Harper, who operates the Once in a Blue Moon website obliquity.com/astro/bluemoon.html, if there’s a full moon on Halloween, it has to be a blue moon.
Since moon phases cycle around 29½ days, a full moon on Halloween means there had to be another full moon earlier in October. David says in North America, we can expect a 100% full moon on Halloween in 2020, 2039, 2058, 2077 and 2096.
In the Middle Ages, scientists and philosophers believed a full moon caused seizures, fever and rheumatism. Because of the connection between the moon and unusual behavior, the afflicted were called lunatics—derived from the Latin “lunaticus,” which translates as “belonging to the moon.”
Much rarer than a blue moon is a month with no full moon.
According to moon fact fan Keith Cooley, home.hiwaay.net/~krcool/Astro/moon, it can only happen in February. In the 21st century, the only occurrences are 2018, 2037, 2067 and 2094.
Even on Halloween—when the moon is a day or two away from completely full—it can still serve as a spooky backdrop because most cannot tell the difference between a 98% illuminated moon and a full moon.
The Sunriver Nature Center and Observatory in Sunriver, Oregon, offers nighttime observatory viewing with advance reservations and required face coverings on select days. More information is available at snco.org.
Visit the Lunar Reconnaissance Orbiter Camera’s website at
lroc.sese.asu.edu to learn more about the moon. The system of three cameras mounted on the Lunar Reconnaissance Orbiter captures high-resolution photos of the lunar surface.
The McDonald Observatory at the University of Texas at Austin has offered monthly virtual moon tours, solar tours and deep sky tours during the pandemic. The plan is to continue them after reopening. Check out the observatory’s YouTube page at youtube.com/channel/UCCdhWitb48ft1HnNRwY-WvQ.
While many astronomy groups have been forced to cancel events due to COVID-19, some have gotten creative about viewing. Volunteer members of the Las Vegas Astronomical Society broadcast live monthly virtual star parties over Zoom and Facebook Live using telescopes with astrophotography cameras. Watch them at lvastronomy.com.
To find an astronomy group near you, visit go-astronomy.com/astro-club-search.htm.
To track the moon phases near you, plug your ZIP code into the database at almanac.com/astronomy/moon/calendar.
If you’re looking for a moon phase calculator with data on phases from 1951 to 2039, visit the one